1. Field of the Invention
The present invention relates to computer systems and, in particular, to improve internal data transfer rates of disk drive type of memories for use with computer systems.
2. Description of the Prior Art
Computers commonly include disk storage systems for secondary memory storage, having moving media on which data can be read or written to. Such disk storage systems are commonly referred to as disk drive units.
Typically, magnetic or optical disk drive units incorporate single or stacked commonly rotated disks for storing data on the surfaces. Data is recorded sequentially on circular data information tracks spaced radially apart on each surface. A magnetic transducer or an optical head reads or writes the data on each disk surface. The process of reading or writing such data to disks is a sequential process. A particular disk surface and track is selected and data is written to or read from the track. After this another track and/or surface is selected and the process is repeated.
A significant time loss, known as "cylinder skew" or "seek time", occurs when moving the head(s) between data tracks in different geometrical cylinders. In order to avoid cylinder skew time loss, data is frequently organized sequentially on tracks that are located in the same geometrical cylinder but on different surfaces.
Another time loss, known as "head skew", occurs when switching from a track on one surface to the next in the same cylinder. In practice the starting points of these surface data tracks are offset by several sector data units to account for this.
Disk drive units may have additional surfaces and tracks, referred to as "servo surfaces, tracks and heads", for controlling the positioning of the data heads over the data tracks. This invention is not concerned with servo control.
In part because of the sequential nature of the process and track and head skew delay, the data transfer rate to and from the disk surfaces is considerably slower than main computer memory.
To improve the data transfer rate from or to the disk surfaces, industry standard disk drive interfaces like "Integrated Drive Electronics" (IDE) and Small Computer System Interfaces (SCSI) frequently incorporate larger on-board buffer or cache memories which can hold several tracks of data read from the disk surface or written to it. This can significantly improve peak data transfer rates. However the actual average and continuous transfer rate from disk drives is still limited by the drive mechanism and its sequential data reading and writing method, head skew and track skew delays.
A significant performance increase has been developed using an array (a plurality) of disk drive units with coordinating external disk rotation electronics. An example of that is the Micropolis Corporation Parallel Drive Array, Model 1804 SCSI, that uses four parallel synchronized disk drive units and one redundant parity drive. Another example is that of U.S. Pat. No. 4,722,085 where 39 disk drive units are used to achieve a 32 bit parallel transfer rate--the other 7 being for error correction. Examples of other efforts in the array method are U.S. Pat. Nos. 4,914,656 and 4,974,156. These have mainly tried to to improve error detection and correction.
The array method is cumbersome both in physical size and the additional external coordinating electronics required. These factors make manufacturing costly for small business and personal computer market. The method does not address the head skew and cylinder skew problems inherent in individual drive units.